Field of the Invention
The present invention relates to a friction plate provided with a core plate of a multiple-disc clutch used for a power transmission portion of automatic transmission for vehicles and a method for manufacturing the friction plate. The present invention relates, more specifically, to a method for subjecting a spline portion of a friction plate in a multiple-disc lockup clutch for a torque converter for an automatic transmission to a laser hardening.
Description of the Related Art
Hardening methods using heat treatments have been conventionally used to enhance wear resistance of a spline portion of a core plate, formed of a low carbon-steel material, of a friction plate for a wet-type multiple-disc clutch.
As typical heat treatment methods that a spline portion of a core plate is subjected to, high frequency, carbon nitriding and the like are utilized. However, these heat treatment methods have been unsatisfactorily performed fine control in selecting a hardening portion of spline portion, setting a hardening depth and the like.
On the other hand, heat treatments are generally performed in a state where a certain number of core plates, each of which is a thin plate and is used for a friction plate, are stacked and fixed, in consideration of deformation after the heat treatments. However, such treatments have a disadvantage that a separating process is necessary for welding caused after the heat treatments since the core plates are stacked and fixed.